crypto: nx - remove 842-nx null checks

Remove the null checks for tfm, src, slen, dst, dlen; tfm will never
be null and the other fields are always expected to be set correctly.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>